Browsing by Subject "Oligo-thiophene"

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Open Access
Nature of charge carriers in long doped oligothiophenes: the effect of counterions
(2008) Zamoshchik, N.; Salzner, U.; Bendikov, M.
A series of oligothiophene dications doped with Cl3 - ions were studied using density functional theory (DFT) at the B3LYP/6-31G(d) level. The balance between the bipolaron and polaron pair states was addressed by studying the closed-shell singlet, open-shell singlet, and triplet states of oligothiophene divalent salts using the relative energies of the different isomers, differences between the singlet and triplet states, bond length alternation analysis, charge distribution analysis, and isodesmic reactions. We found that contribution of polaron pair state to the electronic structure of the oligothiophene divalent salts is not observed in 8T2+(Cl3́ -)2 (short oligothiophene salts), appears in 12T 2+(Cl3 -)2 (medium-sized oligothiophene salts), and becomes the dominant in 20T2+(Cl 3 -)2 (long oligothiophene salts). Bipolarons are intrinsically unstable with respect to dissociation into polaron pairs regardless of the presence of counterions. Thus, even in the presence of counterions, we did not observe any bipolaron stabilization energy, however, doping ions localize polarons. The singlet and triplet states are energetically degenerate for long oligothiophene divalent salts, such as 20T 2+(Cl3 -)2. © 2008 American Chemical Society.
Open Access
Theoretical investigation of excited states of oligothiophene anions
(2008) Alkan, F.; Salzner, U.
Electron-hole symmetry upon p- and n-doping of conducting organic polymers is rationalized with Hückel theory by the presence of symmetrically located intragap states. Since density functional theory (DFT) predicts very different geometries and energy level diagrams for conjugated π-systems than semiempirical methods, it is an interesting question whether DFT confirms the existence of electron-hole symmetry predicted at the Hückel level. To answer this question, geometries of oligothiophene anions with 5-19 rings were optimized and their UV/vis spectra were calculated with time-dependent DFT. Although DFT does not produce symmetrically placed sub-band energy levels, spectra of cations and anions are almost identical. The similarity in transition energies and oscillator strengths of anions and cations can be explained by the fact that the single sub-band energy level of cations lies above the valence band by the same amount of energy as the single sub-band level of anions lies below the conduction band. This and the resemblance of the energy level spacings in valence bands of cations to those in conduction bands of anions give rise to peaks with equal energies and oscillator strengths. © 2008 American Chemical Society.